High frequency fuse

ABSTRACT

A high frequency fuse is provided which closely approximates a transmission line. The fuse consists of a ceramic substrate having electrical conductors deposited thereon which are insulated from each other and a fusing element across the gap between the electrically insulated conductors. The fusing element may be the width of the conductors on the substrate or it can be narrow to obtain the desired fuse clearing current.

United States Patent 1191 1111 3,875,542 Holland et al. 5] Apr. 1, 1975 [54] HIGH FREQUENCY FUSE 3,445,798 5/1969 Lohrmann 337/166 1751 Kenneth Craig Holland, Portland: 32831333 511333 11159711.??? 3521 11; 301mm Hillsbofo, both 3:528:O48 8/1970 Kirk 337/297 Oreg- 3,568,122 2/1971 Urani 337/166 [73] Assignee: Tektronix lnc., Beaverton, Oreg. FOREIGN PATENTS OR APPLICATIONS 22 Filed; Sept 10 97 1,813,430 6/1970 Germany 337/297 867.090 5/1961 United Kingdom .1 337/297 121] App]. No.: 395,666

Rdaed Applicamm Data Primary Examiner-Alfred E. Smith Assistant Examiner-Wm. H. Punter [63] Continuation-impart of Ser. No. 338,013, March 3, 1973. which is a continuation-in-part of Ser. No. Attorney Agent Flrm Adnan La Rue 114,273, Feb. 10, 1971, Pat. No. 3,753,170.

[57] ABSTRACT [52] U.S. Cl. 333/97 R, 337/297 A high frequency fuse is provided which closely ap- [51] Int. Cl. l-l0lp 1/00, H0111 85/04 proximates a transmission line. The fuse consists of a 1 ld Of Search 333/84 M, 97 R; 337/187, Ceramic substrate having electrical conductors depos- 337/295, 297. 216, 207, 201 ited thereon which are insulated from each other and a fusing element across the gap between the electri- 156] References Cited cally insulated conductors. The fusing element may be UNITED STATES PATENTS the width of the conductors 0n the substrate or it can 2.111 1,010 9/1952 Sass ct a1. 333/97 be arrow the deslred fuse clearmg current 3.179.854 4/1965 Luedicke et a1. 317/101 C 2 Claims, 7 Drawing Figures PATENTEUAPR I I SH'LET 1 BF 2 PRIOR ART HIGH FREQUENCY FUSE CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of U.S. Pat. application Ser. No. 338,013 filed Mar. 3, 1973, which in turn, is a eontinuation-in-part of U.S. Pat. application Ser. No. 114,273 filed Feb. 10, 1971, now U.S. Pat. No. 3,753,170.

BACKGROUND OF INVENTION In prior art, fuses have restricted the useful bandwidth of the equipment they were protecting by representing an impedance discontinuity in strip transmission lines. Careful etched circuit board design allowed typical reflection coefficients of approximately percent in a 140 picosecond TDR system and a frequency response of about 2 gigahertz. In addition, the construction of prior art fuses catfid them to be ineffective at higher frequencies because of shunt paths carrying significant amounts ofcurrent. This limited the ability of the fuse to protect the circuits involved.

SUMMARY OF INVENTION The present invention overcomes the disadvantages of the prior art by providing a high frequency fuse element on a ceramic substrate which closely approximates a transmission line. Accordingly, reflection coefficients of less than 2 percent in a 140 picosecond risetime TDR system and a frequency response in excess of 7 gigahertz are obtainable. In addition, higher bandwidths are obtainable using etchant techniques.

It is therefore an object of the present invention to provide a new and improved high frequency fuse which overcomes the disadvantages of the prior art.

It is another object of the present invention to provide a new and improved high frequency fuse which approximates a transmission line.

It is yet another object of the present invention to provide a new and improved high frequency fuse allowing high frequency design specifications.

It is an additional object of the present invention to provide a new and improved high frequency fuse which is effective at high frequencies in protecting associated equipment.

The foregoing and numerous other objects, advantages. and inherent functions of the present invention will become apparent as the same is more fully understood from the following description, which describes the present invention; it is to be understood, however, that these embodiments are not intended to be exhausting not limiting of the invention but is given for purposes of illustration in order that others skilled in the art may fully understand the invention and principles thereof and the manner of applying it in practical use so that they may modify it in various forms, each as may be best suited to the condition of the particular use.

DESCRIPTION OF DRAWINGS FIG. I is a drawing of a high frequency fuse according to the prior art;

FIG. 2 is a drawing of one embodiment of the high frequency fuse according to the present invention;

FIG. 3 shows a second embodiment and the method of making the fuse according to the present invention; and

FIG. 4 is another embodiment of the fuse according to the present invention.

DESCRIPTION OF INVENTION Referring to the drawings, and in particular FIG. 1, there is shown a fuse assembly 1, commonly referred to as a microfuse, and adapted for either plug-in or pigtail installation on printed circuit boards. The microfuse consists of a fuse body 2 having leads 3 and 4 passing therethrough. One end of leads 3 and 4 are used to mount the microfuse onto a circuit board. The other end of leads 3 and 4 are connected together through a fuse wire 5. Plastic is deposited over the connection point of lead 3 and fuse wire 5 and lead 4 and the fuse wire 5 at points 6 and 7. This seals leads 3 and 4 to fuse body 2. A plastic or metal cap 8 is then bonded to fuse body 2 to complete the assembly. As is well-known, these microfuses cause impedance discontinuity in strip transmission lines used with etched circuit boards. As a result of this fuse technique, bandwidth limitations of the equipment being protected exist.

Referring now to FIG. 2, there is shown an embodiment ofa high frequency fuse according to the present invention. In this particular embodiment an epoxy glass circuit board 20 having, say, a gold plated copper layer 21 has an interrupted transmission line 22 etched therein. Transmission line 22 is interrupted in the area 23 over which the high frequency fuse of the present invention will be positioned to make transmission line 22 uninterrupted as well as protecting the associated equipment. The transmission line is fully described in U.S. Pat. application Ser. No. 114,273 filed Feb. 10, 1971, now U.S. Pat. No. 3,753,170 as previously mentioned in the Cross Reference. Mounting post 24 and 25 are carried by circuit board 20 and are used to secure the high frequency fuse to the circuit board. Mounting posts 24 and 25 along with spring clip 26 are fully described in U.S. Pat. application Ser. No. 338,013 filed Mar. 3, 1973 as previously mentioned in the Cross Reference.

The high frequency fuse assembly 30 consists of a ceramic body 27, which is preferably made of alumina. Plated thereon ceramic body 27 is another interrupted transmission line 28. Transmission line 28 is interrupted in the area 31 over which the high frequency fuse wire 29 is secured using ordinary techniques. The fuse wire 29 can be the width of transmission line 28, it can be narrower than the width of transmission line 28, or for that matter, any configuration to obtain the desired fuse-clearing current. The entire assembly 30 is then placed over mounting posts 24 and 25 which pass through guides 33 and 34. Spring clip 26 is then placed to engage grooves in mounting posts 24 and 25 to secure the high frequency fuse to the circuit board.

The above described high frequency fuse provides an unique approach in reducing impedance discontinuity in transmission lines and as a result, bandwidth limitations of the equipment being protected are reduced considerably.

Shown in FIG. 3 is another fuse and method of making the assembly 30 of FIG. 2. In FIG. 3A a substrate 50, such as silicon, has deposited thereon a conduction layer 51. Layer 5] could be, for example, gold. Layer 51 is deposited in a conventional manner. Subsequent masking and etching of layer 51 provides the unit shown in FIG. 3B. The particular shape or pattern of layer 51 provides the desired fuse-clearing current.

FIG. 3C shows the unit of FIG. 38 after a next con trolled back etching of the substrate 50. The back etching provides a beam lead unit 53 which protrudes beyond substrate 50. Beam lead unit 53 can be soldered or welded, say, to the transmission line 28 of FIG. 2. To achieve lower fuse-clearing currents, a further selective back etching of the substrate 50 is performed. This selective back etching leaves a suspended metal bridge in the area 55 which is completely void of substrate 50. Thus, heat sinking of the fuse by the substrate is greatly reduced. The suspended bridge can best be seen in FIG. 3D in which is a cross-section view of the assembly taken along the line CC of FIG. 3C.

Shown in FIG. 4 is another practical embodiment of the fuse assembly for use in place of assembly 30 in FIG. 2. In this embodiment a very short piece of wire or ribbon wire 60 is placed across an interrupted transmission line 62. The wire 60 is secured by conventional techniques. Transmission line 62 passes to the reverse side of a support member 66 through the passages 64 and 65. A protective cover 67 is placed over wire 60 and attached to support member 66. To prevent the wire or ribbon wire 60 from coming into contact with the support member 66 due to expansion or drooping, the support member can be grooved at an an angle perpendicular to the interrupted area of the transmission line. The entire unit is mated with the mounting posts 24 and 25 of FIG. 2 as previously discussed. Using this configuration, the fuse wire 60 is protected at all times.

The use of the high frequency fuse according to the present invention therefore allows narrower specifications on reflection coefficient and allows faster risetime specifications by matching microstrip transmission line impedances.

While there has been shown and described the preferred embodiment of the present invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. For example, the metal used for the fuse of the present invention is preferably gold. However, many other metals could be uti' lized by those skilled in the art.

In addition, it is possible to more nearly approximate the transmission line with the fuse when the interrupted transmission gap is quite large by placing a ground plane at right angles to, and centered within, the gap. Therefore, the appended claims are intended to cover all such changes and modifications that fall within the true spirit and scope of the invention.

The invention is claimed in accordance with the following:

l. A removable high frequency fuse, comprising:

a support member means which has securing means therein and opening means therein;

interrupted transmission line means on opposite sides of said support member means and extending through said opening means substantially intermediate of said securing means; and

a fuseable element that spans said interrupted transmission line means on one side of said support member and is bonded to said interrupted transmission line means, said fuseable element substantially maintaining said transmission line means.

2. A high frequency fuse, comprising:

an electrical mounting member having transmission mediums divided into portions electrically insulated from each other on an insulator member, said transmission mediums defining strip-line transmission lines;

an additional electrical mounting member having transmission mediums divided into portions electrically insulated from each other on an insulator member and having fuse means connected across said divided portions; and

mounting means to removably secure said additional electrical mounting member to said electrical mounting member, said mounting means defining post means carried by said electrical mounting member and extending outwardly therefrom and spring clip means which are disposed into grooves provided in said post means for removably securing said additional electrical mounting member onto said electrical mounting member. 

1. A removable high frequency fuse, comprising: a support member means which has securing means therein and opening means therein; interrupted transmission line means on opposite sides of said support member means and extending through said opening means substantially intermediate of said securing means; and a fuseable element that spans said interrupted transmission line means on one side of said support member and is bonded to said interrupted transmission line means, said fuseable element substantially maintaining said transmission line means.
 2. A high frequency fuse, comprising: an electrical mounting member having transmission mediums divided into portions electrically insulated from each other on an insulator member, said transmission mediums defining strip-line transmission lines; an additional electrical mounting member having transmission mediums divided into portions electrically insulated from each other on an insulator member and having fuse means connected across said divided portions; and mounting means to removably secure said additional electrical mounting member to said electrical mounting member, said mounting means defining post means carried by said electrical mounting member and extending outwardly therefrom and spring clip means which are disposed into grooves provided in said post means for removably securing said additional electrical mounting member onto said electrical mounting member. 